An immunohistochemical procedure employing the chromogenic substrate 3,3' diaminobenzidine tetrahydrochloride (DAB) was developed to elucidate the immunologic interrelationships of peroxidatic catalases in normal peripheral blood leukocytes, leukocytes of patients with myelogenous leukemia and normal erythrocytes. This techniques disclosed that peroxidatic catalase may be specifically identified when immunologic tests are conducted on agarose gel in the presence of DAB, a hydrogen donor. The latter is oxidized to form an insoluble reaction product by the peroxidatic action of catalase reacting on the substrate H2O2. Our data revealed that catalase in peripheral blood leukocytes are myelogenous leukemia and becomes identical to erythrocytic catalase, thus losing some of the physical-chemical characteristics of normal leukocytic catalase.